The present invention relates to an image reading unit for use in an image scanner for reading images, and more particularly to an image reading unit which outputs an image reading signal in the form of a digital signal.
In general, in an image scanner for optically scanning an original document to convert the image of the original into an electrical signal, a main scanning operation in one direction is carried out with an image reading unit having light-detecting elements arranged in a line, while the original or the image reading unit is moved in a direction perpendicular to the main scanning direction to carry out auxiliary scanning, whereby electrical signals are outputted in correspondence to the quantities of light applied to the light-detecting elements to read the image.
In the image reading unit of the image scanner, a light source, such as a fluorescent lamp, for illuminating the surface of the original, an optical lens group acting as a reducing optical system for forming the image of the original irradiated by the light source on the light-detecting element surface, and light-detecting elements for converting the optical image into electrical signals are arranged in that order.
A charge-coupled element (CCD) capable of providing a large output voltage and requiring only a simple read control operation has heretofore been extensively utilized for the light detecting elements in such a scanner. The image signal produced by a CCD sensor is in the form of an analog signal.
The analog signal has typically been transmitted from the image reading unit to the image scanner body through a signal cable without modification. In the image scanner body, the analog signal is either applied to an external device directly or applied to an image processing device or the like after being subjected to digital processing such as analog-to-digital conversion.
In the above-described method of transmitting the image signal in the form of an analog signal from the image reading unit to the image scanner body, however, the image signal is liable to be affected by external noise while passing through the signal cable extending between the image reading unit and the image scanner body. The image signal is thereby lowered in linearity with respect to the original by such noise, and at worst the image signal can be buried in the noise.
The noise is present due to the fact that the signal cable is laid about in the image scanner body. Furthermore, in the case where a CCD sensor is employed as the light-detecting element array, a clock signal for operating the CCD sensor is applied to the CCD sensor from the image scanner body. Since the wire carrying the clock signal and the wire carrying the analog image signal are often laid adjacent to each other in the signal cable, the image signal is greatlY affected by the induced clock noise.